Anaplasma platys (formerly Ehrlichia platys), an obligatory intracellular bacterium, was first described as a rickettsia-like agent in the platelets of dogs from Florida with infectious canine cyclic thrombocytopenia (ICCT) in 1978.25 Authors pointed out morphological and biological similarity of this bacterium to Ehrlichia canis in infected dogs and Anaplasma marginate in infected cattle, two members of the family Anaplasmataceae,25 which were well-known at that time. Clinical signs of ICCT are fever, depression, appetite loss, anorexia, and bleeding tendencies.22 Parasitemia and thrombocytopenia occur in cycles at approximately 10 to 14 day intervals.22 Anaplasma platys responds well to doxycycline, a tetracycline antibiotic, as the primary means of treatment.
Based on indirect fluorescence antibody (IFA) tests using the platelet-rich plasma from a dog experimentally infected with ICCT, minimal serologic cross-reaction was found to occur between A. platys and E. canis, and the researchers proposed the name “Ehrlichia platys” for this bacterium.22 In 1992, the 16S rRNA gene sequence of A. platys was reported.3 Subsequently, the groEL gene sequence of A. platys was disclosed.29, 67 Phylogenetic analysis of these sequences showed that this is a distinct bacterium closely related to Anaplasma phagocytophilum and Anaplasma marginate, which led to reclassification of this bacterium into the genus Anaplasma.17 Later it was reported that although A. platys does not cross-react with serum antibodies from dogs infected with E. canis on IFA tests, the A. platys antigen cross-reacts with anti-Anaplasma phagocytophilum antibodies.32 
Seropositive dogs have been found in Florida, Pennsylvania, Texas, Louisiana, Illinois, California, Arkansas, Mississippi, Idaho, and North Carolina. High rates of A. platys and E. canis dual positive dogs have also been reported throughout these areas.22 A. platys DNA has also been detected in dogs throughout Brazil,20 Greece,43 France,33 Taiwan,15 Spain,54 China,28 Australia,12 Portugal,13 the Democratic Republic of Congo,55 Japan,61 Thailand,29 and Venezuela.59 It is believed that the brown dog tick, Rhipicephalus sanguineus, is the biological vector which transfers A. platys to potential hosts. In fact, A. platys has been detected in brown dog ticks in Okinawa, Japan,34 Spain,58 and the Democratic Republic of Congo.55 However, it has not been experimentally proven that R. sanguineus is the biological vector responsible for the transfer of A. platys.56 To date, A. platys has never been culture isolated. Consequently this bacterium is poorly understood at the molecular, cellular, or immunologic level, and to date, no antigen has been identified for this bacterium.
In A. phagocytophilum and A. marginate, surface-exposed immunodominant 44 kDa major outer membrane proteins (P44s/Msp2s) are encoded by the p44 (msp2) polymorphic multigene family.6, 9, 39, 41, 69-71 In A. phagocytophilum, P44 proteins consist of a single central hypervariable region of approximately 94 amino acid residues and an N-terminal and C-terminal conserved regions of approximately 186 and 146 amino acid residues, respectively.41 A single polymorphic p44/msp2 expression locus (p44/msp2ES) is found in the genome of A. phagocytophilum10 and A. marginale,26 respectively. Both expression loci are found downstream of tr1 genes encoding putative transcriptional factor and homologs of Ehrlichia chaffeensis omp-1 genes encoding polymorphic major outer membrane protein (MOMP).6, 8, 39 At p44/msp2ES, p44s and msp2 donor sequences elsewhere in the genome undergo recombination via RecF pathway to allow variable p44/msp2ES expression under the same promoter.6, 8, 39, 40 This mechanism is thought to facilitate P44/Msp2 antigenic variation persistent infection and for adaptation to new environments such as transmission between tick and mammalian hosts.7, 11, 38, 40, 65, 71 Purified native P44 from A. phagocytophilum and purified native OMP-1s (P28 and OMP-1F) of Ehrlichia chaffeensis have porin activity.30, 37 
Anaplasma platys (Apl) is an obligate intracellular bacteria that infects platelets and causes a cyclic thrombocytopenia in the dog. The observation than a dog can be affected by this rickettsia′ agent, and the disease is most likely transmitted by the Rhipicephalus spp of ticks. Anaplasma platys was first reported in the United States in 1978 and has since been reported in Europe, Asia, South America, the Middle East, Australia, and Africa. Because of the common vector, Anaplasma platys infection is often found as a coinfection with Ehrlichia canis. The ability of the organism to produce clinical disease in the dog appears to vary with geography, suggesting that strain differences may contribute to virulence. Anaplasma platys is related to another Anaplasma species known to cause clinical disease in the dog, Anaplasma phagocytophilum (Aph).
Current diagnostic tests that attempt to distinguish Aph and Anaplasma platys have limited specificity. PCR for Aph and Anaplasma platys using 16SrRNA has also had problems with specificity. Therefore, assays for specific detection of Anaplasma platys are needed in the art. Additionally, serological tests for Anaplasma platys are also needed.